Deprecate LibraryDiscovery and use LibraryDiscoveryCollector instead

The test coverage removed here was added in #2378263: hook_library_alter() must be manually invoked by users of LibraryDiscovery, and has no test coverage, this add a hook_library_alter:

  public function getLibrariesByExtension($extension) {
-    return $this->collector->get($extension);
+    if (!isset($this->libraryDefinitions[$extension])) {
+      $libraries = $this->collector->get($extension);
+      $this->libraryDefinitions[$extension] = [];
+      foreach ($libraries as $name => $definition) {
+        // Allow modules and themes to dynamically attach request and context
+        // specific data for this library; e.g., localization.
+        $library_name = "$extension/$name";
+        $this->moduleHandler->alter('library', $definition, $library_name);
+        $this->libraryDefinitions[$extension][$name] = $definition;
+      }

Then we removed hook_library_alter() in #2472547: Remove deprecated hook_library_alter() and the test was left testing the static caching in LibraryDiscovery, but without checking whether it was still necessary or not.

Let's do the deprecation here.

Once it's merged, a couple of the methods are redundant, so we should probably deprecate those too, but maybe those in a follow-up.

Errm, needs review.

Not 100% if all deprecations need a test to make sure the deprecation is returned. I feel like we know that feature works.

Generally I think we should not do this, and only test deprecations triggered by actual backwards compatibility logic rather than from within deprecated code that's going to be removed. It takes time to write the test, run them every CI run, then remove them again, and they don't tell us much.